We know there were no people around to tell us about the ancient past. But we have some remains that might give us clues about what it was like if we can piece them all together.
Fossils are evidence of dead plants or animals that lived thousands or even millions of years ago. Fossils might be skeletons, or shells, or footprints of extinct animals, or of animals that still roam the earth. The photo of the shell you saw on the previous page was a fossil of an animal that lived in the ocean a long time ago.